(Adapted from the Applicant's Abstract) The overall goal of this application is to characterize the effects of 1,25(OH)2D3 and other potential regulators of the biosynthesis and secretion of parathyroid hormone and attempt to define their modes of action at a molecular level. Central to this study is the recent observation that both 1,25(OH)2D3 and calcium ions decrease the expression of the PTH gene in bovine parathyroid cells in tissue culture. The specific aims of the experimental plan can be divided into three parts: 1) To expand in vitro studies to include various aspects of the interactions of 1,25(HO)2 vitamin D3, calcium, and glucocorticoids on the biosynthesis and secretion of PTH and how inorganic ions such as phosphate, magnesium and aluminum also may be involved. In parallel with studies on PTH, efforts will examine how the biosynthesis of parathyroid secretory protein and vitamin D3 receptor protein may be affected. The possibility of a functional relationship between parathyroid secretory protein and PTH also will be explored. 2) To characterize further regulatory sequences in the PTH gene. Target sequences involved in protein and receptor binding will be identified using "footprinting and bandshifting" techniques, as well as, transfection studies in which transcription of a reporter gene is directed by the PTH promoter preceded by various portions of the 5'flanking sequence. Where is appropriate, specific DNA binding proteins will be characterized by such techniques as Southwestern blotting, and attempts will be made to purify them by classical, as well as, state-of-the-art techniques. 3) Using the recently cloned chicken cDNA probe, experiments are planned to examine evolutionary sequences in earlier vertebrate forms, particularly fish, and these will be compared with their mammalian homologues.